I have an existing old time mercury thermostat ( Trane Weatherton) which I am replacing with programmable Honeywell RTH6300B. Its very confusing how to connect these wires as the Weatherton thermostat has the following wires:
G- green
O- orange
X2 - White
W - white (X2 and W jumperd)
Y - Yellow
T- Tan
R - Red
B- Blue.

and HoneyWell Thermostat has only these
G, W, "not used" , Y, R, Rc and one at the bottom O/B.

I would appreciate if anyone can tell how do I accomplish.
Thanks,

There are a couple of smat guys, on this site and subject may tell give better info, for now, best I would guess. Red is common from transformer, White-Heat,Yellow-Cool, Green-Fan. Thermostat connecting Red with Green and Yellow, turn on Fan and Compressor.
Pulling off cover of Air Handler,and On Cover(hopefully) have Diagram, Look at colors connected.

Can you elaborate more ?

What type of system do you have? Heat pump, gas furnace, A/C??

How can I tell the difference ? I think it would be Heat pump ( for heat ) and A/C ( for cool)